The present invention relates generally to amplifiers and filters of the type involving transconductance stages that employ field effect transistors and, more particularly, to such circuits concerned with common mode feedback.
The communications industry continues to rely upon advances in semiconductor technology to realize higher-functioning devices serving an increasingly complex communication spectrum. For many applications, realizing higher-functioning devices requires the transmission and reception of signals in potentially noisy environments. At the reception end of such communication, recovering the transmitted signal with a high degree of integrity typically requires filtering the analog signal. Preferably, this filtering occurs before significant amplification of the received signal.
Due to its low sensitivity to noise, signal filtering is often achieved using passive filters based on an LC (inductance/capacitance) ladder approach. Designing this type of passive filter, however, is problematic due to an incompatibility of integrating inductors with conventional circuit integration structures and processes. To overcome this dilemma, the conventional inductor has been replaced in many applications with an inductor-simulating electronic component called a gyrator. Gyrators are typically constructed using transistors and capacitors, each of which is fully compatible with conventional circuit integration structures and processes.
In many applications, these signal filters are also required to manifest a linear response within the bandwidth, a precisely defined stopband rejection, and, in some instances, programmability (tunability) within the bandwidth. In defining filter performance, each of these important aspects has been partially realized using Operational Transconductance Amplifiers (OTAs). OTAs are typically implemented using the gyrator design in a differential amplifier configuration. The programmability of the OTA is addressed by varying its transconductance, which is directly proportional to the bandwidth characteristics and inversely proportional to the capacitance.
For many filters requiring or benefiting from gyrators of this differential-transconductance type, precisely controlling the common mode voltage of the transconductance stages is important. This is particularly true for certain channel select filters such as the channel filter recommended in CDMA IS-95, where filter performance must comply with stringent filter linearity and stopband rejection criteria. Gyrators of this differential amplifier type provide a somewhat balanced output having a common mode voltage (or difference potential) that is a function of potentials of nodes in the vicinity of the output nodes. However, because these potentials are difficult to control, an intolerable variation of the common mode voltage can result, which in turn leads to variation in conductance value and thus variation of the cutoff frequencies. Moreover, to satisfy stringent filter linearity criteria, it is important that the operating frequency range be widely defined and not fluctuate.
These difficulties in such differential amplifier gyrator circuits have been partly overcome by common mode feedback (CMFB) implemented as part of gyrator circuits to maintain the common mode voltage within a reasonable range. In one implementation, for example, relatively balanced outputs are provided by merging the CMFB and the differential gain paths to improve the accuracy of the output balancing. For further information on this type of circuit, and related implementations, reference may be made to the following references: Tat C. Choi, et al., High-Frequency CMOS Switched-Capacitor Filters for Communications Application, IEEE Journal of Solid-State Circuits, Vol. SC-18, No. 6, 652-664, December 1983; J. Haspeslagh and W. Sansen, Design Techniques for Fully Differential Amplifiers, IEEE 1998 Custom Integrated Circuits Conference, 12.2.1-12.2.4; Venugopal Gopinathan et al., Design Considerations for High-Frequency Continuous-Time Filters and Implementation of an Antialiasing Filter for Digital Video, IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, 1368-1378, December 1990; and Mihai Banu et al., Fully Differential Operational Amplifiers with Accurate Output Balancing, IEEE Journal of Solid-State Circuits, Vol. 23, No. 6, 1410-1414, December 1988. Problems remain with these conventional approaches including, for example: inadequacy in control over the common mode voltage, unduly limited CMFB bandwidths, unacceptable linearity, and the need for overly large components (e.g., resistors) that are burdensome in terms of silicon real estate and MOS-type IC processing steps.
Accordingly, there is a need for a signal-filtering approach that addresses the bandwidth linearity and stopband rejection criteria required by an increasing number of applications and that overcomes the above-mentioned problems of the prior art.
According to various aspects of the present invention, embodiments thereof are exemplified in the form of methods and arrangements for improving common mode feedback (CMFB) in differential amplifying-type filters. Such embodiments are useful in connection with integration of such filters as part of IC applications, while providing superior filter linearity and stopband rejection.
One specific implementation is directed to a signal-filtering circuit arrangement, comprising a transconductance cell, and a common mode feedback circuit including MOS-based transistors arranged to minimize loading on the transconductance cell. The transconductance cell has first and second current paths, each passing current between power terminals. The common mode feedback circuit includes a high-impedance circuit configured and arranged to compare a sampled common mode voltage to a reference voltage and to provide common mode feedback to the transconductance cell with minimized loading, and further includes a signal-sampling circuit for sampling the common mode voltage of the transconductance cell using a high impedance isolation arrangement of MOS-type transistors.
In another specific example implementation, the present invention is directed to a signal-filtering circuit arrangement comprising a transconductance cell having two pairs of current paths passing current from a power source terminal to a power drain terminal. One of the pairs of current paths is coupled to a differential input port, and another of the pairs of current paths is coupled to a differential output port. The signal-filtering circuit further includes a common mode feedback circuit including a high-impedance circuit and a signal-sampling circuit. The signal-sampling circuit samples a common mode voltage of the transconductance cell at the differential output port using a high impedance isolation arrangement of MOS-type transistors and provides a sampled common mode voltage for driving an input port of the high-impedance circuit. The high-impedance circuit compares the sampled common mode voltage to a reference voltage and provides a common mode feedback to the transconductance cell. More specific example implementations are directed to this same type of signal filtering circuit with following additional aspects: the high-impedance circuit is implemented using NMOS transistors; the high-impedance isolation arrangement of MOS-type transistors is implemented using a source follower arrangement and using a pair of MOS transistors interconnected at a node that is adapted to provide the common mode feedback to the transconductance cell; the transconductance cell includes a common mode feedback input port adapted to control the current paths coupled to the differential output port; the transconductance cell includes a common mode feedback input port adapted to control the current paths coupled to the differential output port; and wherein the signal signaling circuit further includes a plurality of transconductance cells arranged consistent with the above-characterized transconductance cell and with common mode feedback being provided consistent with the above-characterized common mode feedback.
The above summary is not intended to provide an overview of all aspects of the present invention. Other aspects of the present invention are directed to alternative arrangements and methods, as are exemplified and described in connection with the detailed description.